Functional Programming in JavaScript (Node.js)

This book will be your reliable guide into the world of functional programming (FP) on the Node.js platform. Unlike many abstract guides, the author offers a practical approach, relying on real-world web development tasks: from data stream processing to building fault-tolerant microservices.

Who is this book for?

• Junior and Middle level JavaScript developers looking to improve code quality and get rid of "callback hell" and side effects.
• Node.js backend engineers striving to master a declarative programming style for creating predictable and easily testable systems.
• Students and graduates of IT courses who want to go beyond OOP and understand alternative paradigms.
• Technical leaders seeking ways to reduce codebase complexity in large projects.

What will you learn from the book?

• Fundamental principles: pure functions, data immutability, higher-order functions, and closures.
• Advanced techniques: currying, partial application, function composition, and recursion (including tail recursion).
• Working with monads (Maybe, Either, IO) and functors using the Ramda library and built-in JavaScript capabilities.
• FP patterns in asynchronous code: stream processing (RxJS), functional middleware in Express/Koa.
• Testing pure functions and integrating FP with TypeScript.
• Refactoring a real project by transitioning from an imperative style to a functional one.

The book is structured on the "from simple to complex" principle: each chapter contains a theoretical block, code examples (available for download), and exercises for independent work. Special attention is paid to performance — you will learn when FP speeds up code and when a different approach should be applied.

In the final part, an end-to-end example awaits you: creating a REST API for an analytics service, built entirely on functional principles — from models to routing. By the end of the reading, you will be able to confidently apply these concepts in commercial development, making your code more concise, reliable, and easy to maintain.

Whether you are working on a startup or maintaining a legacy system, the skills of functional thinking will change your approach to programming forever.